The present invention relates to an objective lens for pickup in an optical recording apparatus which conducts recording on or reproducing from an information recording medium such as an optical disk, and to a optical pickup apparatus employing the objective lens.
In an optical system of an optical recording/reproducing apparatus employing a medium of an optical disk, there is commonly used an aspherical single objective lens. For achieving high density of recorded information signals, a size of a spot formed on a recording medium by the objective lens has been required to be small, and there have been investigated high NA of an objective lens and utilization of a light source for short wavelength.
Though there has been developed a GaN blue semiconductor laser having an oscillation wavelength of about 400 nm, a wavelength is varied by mode hop or by laser output, and monochromaticity of oscillation wavelength is poor because high-frequency superimposition is used. Therefore, in a light-converging optical system for high density optical disk wherein a GaN blue semiconductor laser is used, it is considered that correction of axial chromatic aberration is necessary.
In an aspherical single objective lens for an optical disk, spherical aberration and coma are corrected by aspherical surfaces. However, when a numerical aperture is large, image height characteristics are deteriorated. When an optical disk is made to be of higher density, deterioration of the image height characteristics becomes an extremely serious problem even if the value of the deterioration is small. In particular, when the numerical aperture is 0.65 or more, the problem is conspicuous. Further, when the numerical aperture is greater, deterioration of eccentricity sensitivity is also a serious problem.
The invention has been achieved for solving the problem stated above. Namely, with regard to an objective lens for a optical pickup apparatus, an object of the invention is to provide an aspherical single objective lens whose numerical aperture is great and image height characteristics are excellent. In particular, an object of the invention is to provide an objective lens which is suitable to be used for a high density recording/reproducing apparatus wherein a numerical aperture is not less than 0.65, preferably 0.7 or more, and more preferably 0.75 or more, and there is used a laser wherein a wavelength of a light source is as short as about 500 nm.
Further, providing an objective lens which makes eccentricity sensitivity to be excellent is also an object of the invention. Furthermore, providing an objective lens which makes spherical aberration and coma to be excellent is also an object of the invention.
When a thickness of a protective layer (transparent base board) of an information recording medium is small to be 0.2 mm or less, or when there is no protective layer, a working distance can be small. An offer of an objective lens which is suitable to be used in a recording/reproducing apparatus having such small working distance is an object of the invention.
It is further an object of the invention to provide a optical pickup apparatus, an optical information recording medium recording/reproducing apparatus, and an optical information recording. medium recording/reproducing method, which employ these objective lenses stated above.
Further, an object is to provide a optical pickup apparatus having an optical system wherein axial chromatic aberration is corrected by the simple structure, in a high density optical recording/reproducing apparatus. In particular, it is an object to provide a optical pickup apparatus wherein a numerical aperture on the part of an information recording medium is 0.65 or more, preferably 0.7 or more, and more preferably 0.75 or more, and shortest wavelength of a light source to be used is as small as 500 nm or less.
The above object can be attained by the following structures.
(1) An objective lens for use in an optical pickup apparatus to record or reproduce information in an optical information recording medium, comprising:
an aspheric surface;
wherein the following conditional formula is satisfied:
xe2x80x831.1xe2x89xa6d1/fxe2x89xa63
xe2x80x83where d1 represents axial lens thickness and f represents a focal length.
(2) In the objective lens of (1), a numerical aperture of the objective lens is not smaller than 0.65.
(3) In the objective lens of (2), a numerical aperture of the objective lens is not smaller than 0.75.
(4) In the objective lens of (1), the following conditional formula is satisfied:
f/vdxe2x89xa60.060
xe2x80x83where vd represents Abbe""s number.
(5) In the objective lens of (1), the following conditional formula is satisfied:
1.40xe2x89xa6n
xe2x80x83where n represents a refractive index at a used wavelength.
(6) In the objective lens of (5), the following conditional formula is satisfied:
1.40xe2x89xa6nxe2x89xa61.85
(7) In the objective lens of (1), the following conditional formula is satisfied:
0.40xe2x89xa6r1/(nxc2x7f)xe2x89xa60.70
xe2x80x83where r1 represents a paraxial radius of curvature of one surface of the objective lens.
(8) In the objective lens of (7), r1 represents a paraxial radius of curvature of the surface of the objective lens at the light source side.
(9) In the objective lens of (1), a used wavelength is not longer than 500 nm.
(10) In the objective lens of (1), the objective lens is an objective lens for use in an optical pickup apparatus to record or reproduce information in an optical information recording medium having a protective layer whose thickness is not larger than 0.2 mm.
(11) In the objective lens of (10), a numerical aperture is not smaller than 0.7.
(12) In the objective lens of (7), the following conditional formula is satisfied:
1.50xe2x89xa6n
xe2x80x83where n represents a refractive index at a used wavelength.
(13) In the objective lens of (1), the objective lens is a plastic lens.
(14) In the objective lens of (1), the objective lens is a glass lens.
(15) In the objective lens of (1), the following conditional formula is satisfied:
1.85xe2x89xa6n
xe2x80x83where n represents a refractive index at a used wavelength.
(16) In the objective lens of (1), the objective lens further comprises a diffracting section.
(17) In the objective lens of (1), the objective lens further comprises a flange section on an outer periphery thereof.
(18) In the objective lens of (1), the flange section comprises a surface extended in a direction perpendicular to an optical axis.
(19) In the objective lens of (1), each of both lens surfaces is an aspherical surface.
(20) An optical pickup apparatus to record or reproduce information in an optical information recording medium, comprises:
a light source to emit light flux;
a converging optical system to condense the light flux emitted from the light source; and
an optical detector to detect reflection light from the optical information recording medium;
xe2x80x83wherein the converging optical system comprises an objective lens to condense the light flux on an information recording surface of the optical information recording medium and the objective lens comprises an aspheric surface; and
wherein the following conditional formula is satisfied:
1.1xe2x89xa6d1/fxe2x89xa63
xe2x80x83where d1 represents axial lens thickness of the objective lens and f represents a focal length of the objective lens.
(21) In the optical pickup apparatus of (20), a numerical aperture of the objective lens at the optical information recording medium side is not smaller than 0.65.
(22) In the optical pickup apparatus of (29), a numerical aperture of the objective lens at the optical information recording medium side is not smaller than 0.75.
(23) In the optical pickup apparatus of (20), the following conditional formula is satisfied:
f/vdxe2x89xa60.060
xe2x80x83where vd represents Abbe""s number of the material of the objective lens.
(24) In the optical pickup apparatus of (20), the following conditional formula is satisfied:
1.40xe2x89xa6n
xe2x80x83where n represents a refractive index of the material of the objective lens at a used wavelength.
(25) In the optical pickup apparatus of (24), the following conditional formula is satisfied:
1.40xe2x89xa6nxe2x89xa61.85
(26) In the optical pickup apparatus of (20), the following conditional formula is satisfied:
xe2x80x830.40xe2x89xa6r1/(nxc2x7f)xe2x89xa60.70
xe2x80x83where r1 represents a paraxial radius of curvature of the surface of the objective lens at the light source side.
(27) In the optical pickup apparatus of (20), the light source emits light flux whose wavelength is not larger than 500 nm.
(28) In the optical pickup apparatus of (20), the optical pickup apparatus is used to record or reproduce information in an optical information recording medium having a protective layer whose thickness is not larger than 0.2 mm.
(29) In the optical pickup apparatus of (28), a numerical aperture of the objective lens at the optical information recording medium side is not smaller than 0.7.
(30) In the optical pickup apparatus of (20), the following conditional formula is satisfied:
1.85xe2x89xa6n
xe2x80x83where n represents a refractive index of a material of the objective lens at a wavelength of the light flux emitted from the light source.
(31) In the optical pickup apparatus of (20), the converging optical system comprises a diffracting section.
(32) In the optical pickup apparatus of (20), the converging optical system comprises a coupling lens to change a divergent angle of the light flux emitted from the light source and the coupling lens corrects chromatic aberration of the objective lens.
(33) In the optical pickup apparatus of (32), the coupling lens is a collimator lens to make the light flux emitted from the light source to be parallel light flux.
(34) In the optical pickup apparatus of (32), chromatic aberration of the composite system of the objective lens and the coupling lens satisfies the following conditional formula:
xcex4fbxc2x7NA2xe2x89xa60.25 xcexcm (xcex4fb greater than 0)
xe2x80x83where xcex4fb represents a change of focal position (xcexcm) of the composite system when a wavelength is changed from a standard wavelength by +1 nm, and NA represents a numerical aperture of the objective lens at the optical information recording medium side.
(35) In the optical pickup apparatus of (34), the chromatic aberration of the composite system of the objective lens and the coupling lens satisfies the following conditional formula:
0.02 xcexcmxe2x89xa6xcex4fbxc2x7NA2xe2x89xa60.15 xcexcm (xcex4fb greater than 0)
(36) In thee optical pickup apparatus of (32), the following conditional formula is satisfied:
0.1xe2x89xa6|m|xe2x89xa60.5 (mxe2x89xa60)
xe2x80x83where m represents magnification of the composite system of the objective lens and the coupling lens.
(37) In the optical pickup apparatus of (32), the coupling lens is a single lens group having two lenses.
(38) In the optical pickup apparatus of (32), the coupling lens comprises an aspherical surface.
(39) In the optical pickup apparatus of (32), the coupling lens comprises a diffracting section.
(40) An apparatus to record or reproduce information in an optical information recording medium, comprises:
an optical pickup apparatus, comprising
a light source to emit light flux;
a converging optical system to condense the light flux emitted from the light source; and
an optical detector to detect reflection light or transmission light from the optical information recording medium;
xe2x80x83wherein the converging optical system comprises an objective lens to condense the light flux on an information recording surface of the optical information recording medium and the objective lens comprises an aspheric surface; and
wherein the following conditional formula is satisfied:
1.1xe2x89xa6d1/fxe2x89xa63
xe2x80x83where d1 represents axial lens thickness of the objection lens and f represents a focal length of the objective lens.
(41) A method of recording or reproducing information in an optical information recording medium, comprises:
a step of emitting light flux;
a step of converging the light flux emitted from the light source on an information recording surface of the optical information recording medium; and
a step of detecting reflection light or transmission light of the light condensed on the information recording surface;
xe2x80x83wherein the light flux is converged on the information recording surface of the optical information recording medium by an objective lens; and
wherein the objective lens comprises an aspheric surface and the following conditional formula is. satisfied:
1.1xe2x89xa6d1/fxe2x89xa63
xe2x80x83where d1 represents axial lens thickness of the objective lens and f represents a focal length of the objective lens.